Sinkhole #25 lies in Bitter Lake National Wildlife Refuge northeast of the city of Roswell, Chaves County, New Mexico, near the winding bed of the Pecos River. The very name “Pecos” brings up visions of the early days of the Wild West and the land around the immediate vicinity of Sinkhole #25 certainly does not immediately dispel this illusion. The city of Roswell is not visible—only rolling hills and flatlands that parallel the river as it flows south toward Texas.

Actually the term “flow” is probably not very accurate most of the time as the river is often completely dry over much of its extent. Here the main source of water comes from springs, such as Sago Spring, and local rainfall, and is often stored in sinkholes that have formed in gypsum deposits. The springs also usually keep the river flowing through the Refuge. Sinkhole #25 is but one of the water storage areas. It is not an especially large sinkhole, being nearly circular and possibly around 10 meters in diameter and maybe 4-5 meters deep. The nearby Sinkhole #20 is much larger, deeper (perhaps to 7-8 meters deep) and more irregular.

Shore of Bitter Lake

The pH and salinity of the two sinkholes, based on winter and summer spot sampling since 1997 has varied considerably as follows: Sinkhole 20: pH: 6.67-8.92 salinity¹: 6.9-28.9; Sinkhole 25: pH: 7.49-9.0 salinity: 10.4-19.6. (Data supplied by Gordon Warrick, Biologist, U. S. Fish and Wildlife Service, Bitter Lake National Fish and Wildlife Refuge.) The variation depends on the height of the water and the influx of freshwater from rain and spring water. With a top pH of 9.0, Sinkhole 25 is certainly somewhat more alkaline, but oddly the highest (and lowest) salinity was recorded at Sinkhole #20.

I visited both sinkholes during the summer of 2003 on an expedition to start a survey of certain arthropod groups for the area, but took a small sample of water from Sinkhole #25 on August 6, 2003, to examine under a microscope. At the time this was a bit of an afterthought and I neglected to sample the nearby Sinkhole #20. I thus cannot claim that the sample I took is representative of even the microfauna of that sinkhole, let alone any of the others. However, I did make one interesting observation about the two sinkholes while I was there - the aquatic insect fauna seemed to be different! Sinkhole #20 had numerous beetles in the family Haliplidae, which appeared to be lacking in Sinkhole #25. There also appeared to be more damselfly nymphs in sinkhole #25 than in Sinkhole #20. Again these were not scientific observations, but were food for thought and possible revisits to the sinkholes.

I took the sample of water home in a small glass jar and set it on my microscope table. Busy with other tasks, it took me a while to get back to the jar and by that time three days had passed. However my first look under the phase-contrast microscope was quite encouraging. There were literally millions of ciliates swimming through the drop that I examined. At this point I regretted not taking more samples and having not gotten to this sample earlier! Obviously there were large numbers of bacteria present, but little in the way of algae that I could see. I also could not find any rotifers, but ciliates were everywhere. They ranged from large (200 μm) Spathidium sp., through medium-sized barrel-like Coleps sp. to large numbers of tiny species I could not identify at all. A few Vorticella sp. were attached to bits of the water plant Chara. In addition, there were a few dinoflagellates (Peridium?) swimming along in their characteristically jerky motion. I also saw a phenomenon that I re-observed over the next few weeks. What appeared to be boiling masses of apparently tiny Euglena-like protoctists (thousands) through which the larger species would occasionally swim.

Again I did not get back to the sample for a while (a week this time). Spiny-looking species resembling Caenomoepha were abundant, as were Coleps. I discovered the largest ciliate yet—apparently a species of Paramecium probably over 300 μm in length. There were thousands of tiny ciliates (ca. 25 μm or less) as well as the strange “balls” of protoctists. Two days later a few species were added that I could not identify—one elongated ciliate with a bent “neck” and another with a tail-like structure and easily visible anterior bands of cilia. The “balls” of protoctists were still active.

My last observation, five days later, showed a culture that was starting to decline. The boiling masses were not evident, but I saw a “jumping” ciliate (Halteria sp. or possibly Clyclidium sp.) for the first time. Several nematodes were present and I also observed a medium-sized “folded-over” ciliate I had not seen before.

The numbers and variety of ciliates especially were remarkable. Obviously the gypsum sinkhole, with its salinity and alkalinity was very productive of life. However, the observations that I made were only preliminary and I am not a trained protozoologist. It would be especially productive I think to examine as many sinkholes as possible. The area has extremely diverse floras and faunas, being known for having one of the highest (if not the highest) number of species of odonates (dragonflies and damselflies) in the United States—85 at last count. It would seem that a unique set of circumstances has produced each of these sinkholes and that each may have a quite different microfauna. Each sinkhole, depending on underground or above ground connections to other sinkholes or springs, may well be in part its own universe. I would like to know if anybody knows of studies on such areas or if anyone is familiar with the peculiar “balling” behavior of the tiny protoctists I saw.

I offer my thanks to the officials at Bitter Lake National Wildlife Refuge, who allowed me access to the sinkholes, and especially to the refuge biologist, Gordon Warrick, who guided me to sink hole #25, among others.

Comments to the author, David Richman, are welcomed.

Identifications were made with Patterson, D. J. 1998. Free-Living Freshwater Protozoa: A Colour Guide, John Wiley and Sons, New York, and checked in Kudo, R. R. 1966. Protozology, 5^th ed., Charles C. Thomas Pub., Springfield, IL, and Jahn, T. L., E. C. Bovee and F. F. Jahn. 1979. How To Know the Protozoa, 2^nd ed., William C. Brown, Dubuque, IA.

Footnote 2: The author used 100-200x magnification for his observations.